Components in integrated optics, e.g. couplers and modulators can be included in fibre-optical systems for information transmission. These components include a wafer of opto-electrical material having optical waveguides indiffused into its upper surface. In a coupler, for example, a transmitted light signal can be switched or crossed over to either of the coupler's outputs with the aid of electrodes on the surface of the wafer. The components generally have the disadvantage that light with well-defined polarization is required for this switching to take place satisfactorily. If the polarization state is indefinite the transmitted signal can be split up between the outputs so that a fault in the signal transmission occurs in the coupler. A well-defined polarization state in transmitted light signals can be maintained if the transmission takes place with polarization-maintaining optical fibres, although these attenuate the light signals rather heavily and are expensive. These difficulties in transmission can be avoided by the optical components being made polarization-independent. In Appl. Phys. Lett. 35(10), Nov. 15, 1979, pp 748-750, R. C. Alferness; "Polarization-Independent Optical Directional Coupler Switch using Weighted Coupling" there is proposed a polarization-independent optical directional coupler, in which the distance between the optical waveguides varies in the interaction region of the coupler. This directional coupler has the disadvantage that it requires great accuracy in manufacture of the optical wave guides and electrodes. In the Journal of Lightwave Technology, Vol. LT-2 No. 1, Feb., 1984, Leon McCaughan "Low-Loss Polarisation-Independent Electro-Optical Switches at .lambda.=1,3 .mu.m", a polarization-independent optical coupler is suggested which is somewhat simpler than in the preceding reference, but which has greater crosstalk between the optical waveguides. A further polarization-independent optical coupler is suggested in IEEE Journal of Quantum Electronics, Vol. QE17, No. 6, June 1981, pp 959-964, N. Tsukada and T. Nakayama; "Polarization-Insensitive Integrated-Optical Switches: A New Approach". In this publication there is described a coupler having two sections in the interaction region with two separate types of electrodes. These electrodes are each divided into a large number of smaller electrodes, and the directional coupler is complicated in manufacture. Apart from the above-mentioned disadvantages with large complexity and high demands on manufacturing accuracy, many known polarization-independent optical directional couplers also have the disadvantage that they require high drive voltages. This can be a difficulty in many applications.
The above mentioned problem, that a transmitted light signal has an indefinite polarization state can also be solved by splitting a received lightwave into two signals with orthogonal polarization states, each being processed by itself in a receiver. Apparatus for achieving such lightwave splitting is described in Optical Letters, Vol. 10, No. 4, 1984 pp 140-142, R. C. Alferness and L. L. Buhl: "Low Crosstalk Waveguide Polarization Multiplexer/Demultiplexer for .lambda.=1.32 .mu.m" and in Electronics Letters, Vol. 23 1987, pp 614-616, K. Habara: "LiNbO.sub.3 Directional Coupler Polarization Splitter". Such apparatus includes directional couplers of lithium nobiate having light propagation at right angles to the optical axis of the single crystal wafer pertaining to the coupler. In this crystal orientation the two orthogonal polarization directions, i.e. the TM and TE modes, have different propagation constants in the coupler waveguides. The bar state for one mode and the crossover state for the other mode can be obtained by connecting a suitable voltage to the coupler electrodes. These known apparatus require great manufacturing accuracy, however, for obtaining good splitting of the polarization directions, and the apparatus are therefore expensive.